Capcom has announced that a demo for the story mode in Avengers Marvel vs. Capcom: Infinite is currently available and is free to download for the PS4 and Xbox One. Not sure why it's not available on PC. Anyways the heroes from both sides come together in order to stop Ultron Sigma and the heroes also need help from Thanos, so this is probably one of those "enemy of my enemy is my friend" situations.
They also announced new characters including...
Marvel vs. Capcom: Infinite launches on September 19th 2017 for PS4, Xbox One and PC.